Lability and solubility of Ni, Cu, Zn, Cd and Pb in UK soils

Garforth, Judith. 2015 Lability and solubility of Ni, Cu, Zn, Cd and Pb in UK soils. University of Nottingham, PhD Thesis, 261pp.

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Currently, total metal concentrations (MTotal) are used to assess the toxicity risks of soil metal to human and environmental health. However, not all soil metal is available over the timescales relevant for toxicity, and therefore use of MTotal may overestimate the risks. Measurements of labile soil metal can be used (1) to define critical limits that reflect the amount of metal in the soil that is ‘potentially bioavailable’ to plants and soil organisms that take up metal from the soil solution and (2) as inputs to geochemical models, for the purpose of predicting soil metal solubility, speciation and dynamics, which are also important for risk assessment purposes. Labile soil metal can be measured using isotopic dilution and chemical extraction methods, but these measurements are not widely available for the soils used in toxicity studies, or for the soils included in national scale geochemical surveys. Therefore the objectives of this study were: (1) to assess the current multi-element stable isotopic dilution methodology for measuring labile Ni, Cu, Zn, Cd and Pb in soils (E values), (2) to evaluate how E values compare to alternative measurements of labile soil metal, (3) to calculate how E values relate to soil properties in order to form predictive algorithms for E values and (4) to use E values and geochemical modelling to predict soil metal solubility. E values (for 6 soils) were found to be operationally defined by the isotope spike contact time used in the multi-element stable isotopic dilution method. However, between 3 and 5 days of isotope spike contact time the % E values increased by less than 5 % for all soil-metal combinations tested. The multi-element spike solution caused a significant decrease in the pH, and increase in the solution concentrations of Ni, Cu, Zn, Cd and Pb in the suspensions of one soil, and Cu in all 6 soils. Metal concentrations solubilised by eight commonly used extractants (MExt) were directly compared to the E values measured in four soils. The extractants used were 0.21 M and 0.43 M HNO3, 0.43 M CH3COOH, 0.01 and 0.05 M Na2H2EDTA, 5 % TMAH, 0.55 M NaOH, 0.5 M NaHCO3 and 0.5 M NH4H2PO4. The 0.01 M Na2H2EDTA extractant provided the best overall analogue for E value. The concentrations of isotopically exchangeable metal were also measured under the conditions of the 0.43 M iii HNO3, 0.43 M CH3COOH, 0.05 M Na2H2EDTA and 1 M CaCl2 extractants (combined isotopic dilution-chemical extraction assay; EExt). Comparison of MExt, E value and EExt revealed whether the extractants (1) mobilised any non-labile soil metal and (2) solubilised all extraction-labile soil metal. The only extractant to consistently solubilise all extraction-labile soil metal was 0.05 M Na2H2EDTA, however this extractant also caused mobilisation of non-labile metal in the three out of the four soils. Predictive algorithms, using the soil characteristics log MP-Total (pseudo-total), pH and LOI, explained more than 80 % of the variability in Ni, Cu, Zn, Cd and Pb log E values (109 soils), but only 53 %, 6 %, 47 %, 42 % and 56 % of the variability in Ni, Cu, Zn, Cd and Pb % E values respectively. Predictive algorithms using 0.05 M Na2H2EDTA MExt and pH explained more than 90 % of the variability in Ni, Cu, Zn, Cd and Pb log E values. Soil solution metal concentrations were predicted using the geochemical assemblage model WHAM/Model VII, using either MTotal, MExt or E values as model inputs, and compared to solution concentrations measured in 0.01 M Ca(NO3)2 soil suspensions. Using MTotal as an input generally resulted in overestimation of the soluble concentrations of all five metals, but using E values improved model predictions.

Item Type: Publication - Thesis (PhD)
CEH Sections: Shore
NORA Subject Terms: Ecology and Environment
Agriculture and Soil Science
Date made live: 09 Dec 2015 12:46 +0 (UTC)

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